Field of the Invention
This invention relates to transportation of natural gas, and in particular, it relates to a process of transporting natural gas through solution in liquid hydrocarbon.
Description of the Related Art
The supply of U.S. natural gas is growing at such a rate from so-called unconventional or “shale gas” production, as to eventually warrant U.S. natural gas export. The U.S. presently does not export natural gas offshore, although offshore export of natural gas is an established industry outside of the U.S. 10% of global natural gas production in 2014 was transported via maritime transport, as liquefied natural gas, LNG. Dominant export countries such as Qatar, Malaysia, Indonesia, Nigeria, and Australia, possess large natural gas fields that produce volumes greatly exceeding domestic demand. Natural gas import markets via maritime transport are predominantly in Japan, China, South Korea, India, and Taiwan.
The condition in which U.S. domestic supply exceeds demand is beginning to arise. U.S. natural gas production and consumption dominate global data: in 2014 the U.S. balanced production and consumption at 22% of global totals. Only two other countries, China and Russia, accounted for more than 5% of global production or consumption. A U.S. natural gas oversupply condition may therefore potentially add substantial volumes to international natural gas markets.
Offshore marine transport of U.S. natural gas is therefore being undertaken by U.S. industry and government. Improving the economic efficiency of marine transport of U.S. natural gas is therefore an important effort. This invention contributes to that effort.
Onshore transportation of natural gas occurs almost exclusively by pipeline. Natural gas is not transported by pipeline across large expanses of water however, due to prohibitive pipeline and facilities material and installation cost. Trans-ocean export of natural gas therefore occurs exclusively in discrete shipments by marine vessels.
Pipeline transport is comprised of flowing the gas in a continuous stream from a high-pressure point of origin to a lower pressure point of sales. Over land, flowing a continuous stream through a pipeline is significantly more cost effective than transporting the gas in discrete shipments by tanker truck or rail.
Discrete transport (marine vessel, rail, or tanker truck) is more appropriate for hydrocarbon liquid than for natural gas, because a unit volume of liquid hydrocarbon such as crude oil, gasoline, diesel, or kerosene contains a significantly larger quantity of energy than that of natural gas. To illustrate, at atmospheric pressure and ambient temperature, a similar heating value is contained in 1 cubic foot of crude oil, as in 1000 cubic feet of natural gas. Therefore, 1000 discrete shipments of natural gas at ambient conditions must occur to transport an equivalent energy quantity as 1 similar shipment of crude oil.
Feasible discrete shipping of natural gas therefore requires that a given gas heating value occupy a much smaller volume than that at ambient conditions—its density must be increased. This is presently commercially accomplished at ambient pressure by lowering the temperature of the gas sufficiently that it becomes liquid, or compressing the gas at ambient temperature with sufficient pressure such that although it remains in gaseous form, it occupies a significantly smaller volume. These are presently the two fundamental physical approaches employed to produce Liquefied Natural Gas—LNG, and Compressed Natural Gas—CNG, respectively, for discrete transportation of natural gas.
Presently, LNG is a commercially feasible method of marine transportation of natural gas. CNG to date has not been a commercially viable marine transportation option, despite numerous proposed projects in the previous 50 years.